Thousands of bird species are migratory,
performing a north-south, often cross-equatorial,
seasonal migration between northern and southern
homes (Figure 3.6), of which the one north of the
subtropical region is used for breeding. A migratory
bird is an integral component of these two distinct
ecosystems, linked only by migration routes,
used for reaching its destination in the fastest
and safest way. Since most deserts are wedged
within the subtropical latitudes, since many regions
within subtropical latitudes are deserts, and since
competition for resources at either end favors early
arrival, most migrants select the shortest route,
which is very likely to include a desert-crossing
section serving as a corridor for the migration of
large numbers of birds.
Migrating is always costly, but more so on desert
crossing, because migrants spend most of their
life outside the desert and hence are not as well-adapted to deserts as resident birds. Yet, since
crossing deserts rather than circumventing them
evolved as an adaptation to reduce travel time, the
added cost is reduced by specific adaptations. The
fuel used for travel is fat-light in weight and rich
in energy -stored under the skin. A 10g warbler
needs fat stores approximately a quarter of its body
mass to complete the flight across the Sahara
Desert, which can take 40 hours (Carmi and others
1992). Burning this fat during the intensive and
exhaustive flight generates much heat and the
rapid breathing during flight removes body water.
To reduce the risks of overheating and dehydration,
most small cross-desert migrants fly at night.
When the desert tract to be crossed is short, birds
may make it in a single flight, but otherwise birds
conduct intermittent flights between stopovers
(Biebach and others 2000); they alight at dawn,
seeking shaded and concealed refuges, even as
small as a shading rock or a single bush if nothing
better is in sight, where they minimize water loss
and rest prior to taking off again when night falls.

Migrants can do even better when stoppingover
in desert oases, which function as stepping
stones for en route energy replenishment (Safriel
and Lavee 1988). But because oases are rare (for
example, the combined area of Saharan oases is
only 2 per cent of this desert), scattered and hence
difficult to spot, many migrants use routes along desert rivers such as the Nile (Box 3.1). Combined,
all desert oases and riparian corridors comprise a
critical habitat for a hugely disproportionate number
of migratory species. Yet, the richer an oasis or a
riparian tract is, the more it is likely to be impacted
by people (two-thirds of the human population of
the Sahara concentrate in oases) who can pose a
threat to the migrants; the intensification of irrigated
agriculture in natural and man-made oases, for
example, may add open water and some locally
abundant sources of food, which does attract
migrants, but agrochemicals and other pollutants
may harm the migrants, either directly, or through
damage to the insects they might feed on (e.g.,
Evans and others 2005).

Cross-desert bird migration is also being impacted
by global climate change. Cues such as day length,
which are independent of climate, are involved in
determining the onset of migration. These have
evolved to synchronize with changes in seasonal
abundance of plant and insect food resources
in the two far-apart "homes" of the migratory
birds; in contrast with the day-length cues, these
changes in food availability are highly dependent
on climate. Climate change will decouple the
synchrony between non-climatic cues and climatic
events. This synchrony is critical for cross-desert
migrants, many of which time their migration such
that they arrive at their feeding ground, just prior to entering, or just after exiting the desert when
the food supplies there peak, which enables them
to replenish their fat reserves for the remaining
journey (Vickery and others, 1999). Compounding
that problem is the projected "expansion" of
deserts, which would increase the distance of
that flight, perhaps beyond that which birds are
already adapted to, or to which they might rapidly
adapt. Furthermore, even if climate change does
not jeopardize the voyage itself, it may reduce the
benefits of migration. Since migrants have only a
small margin of safety of energy reserves during
migration, the condition in which they arrive at their
destination, where they may encounter intense
competition, substantially determines their survival.

To conclude, the cross-desert migration of birds,
many of which are both familiar and important
to people living far from deserts, is sensitive to
human impact, climate-change included. Since
this migratory network can only be as strong as its weakest link, the conservation of desert sites used
by alighting migrants, as well as off-desert ones
on which the success of cross-desert migration
depends, is urgently required (hutto 2000).

Locusts moving through deserts

Though there are many more insect species than bird
species, the number of migratory insects is smaller
than that of migratory birds and only one small
group of migratory insects is associated with deserts
- the locust. Unlike migratory birds which cross
deserts in a regular, seasonal two-way migration,
locusts cross the desert in a unidirectional, irregular
pattern. And, whereas the arrival of migratory birds
is often welcome, the sighting of locust swarms is
always ominous. Several locust species spend part
of their lives in deserts and many locust swarms
cross deserts. Most significant is the desert locust
(Schistocerca gregaria) of the least dry parts of
deserts - arid (but not hyperarid) regions in 25
countries of the Sahel (including Burkina Faso, Chad, Mali, Mauritania and Niger), the Arabian Peninsula,
along the coast of the Red Sea, and along the coast
of ROPME Sea Area (Regional Organization for the
Protection of the Marine Environment - Kuwait
Regional Convention, 1978) up to the India-Pakistan
border (Simpson and others 1999). The desert locust
in the solitary phase poses no threat to crops; its
small populations are dispersed in patches of suitable
habitat, with little movement between them (Ibrahim
and others 2000).

When spells of good rainfall occur in several
successive rainy seasons, the soil becomes
moister and vegetation grows more quickly. In
response the animals also grow more quickly
and egg-laying in holes dug in the soft, moist
soil intensifies. Each of the formerly isolated
populations increases, and their movement
downwind leads to a concentration of several
crowded, fast-growing populations (Despland and others 2004). Once the aggregated population
is large and crowded, the individuals change in
colour, physiology and behaviour which, combined,
helps them to aggregate and reproduce intensively
(Pener 1991) especially when rains persist,
thus increasing pressure on resources. When,
eventually, vegetation is decimated at the source,
a large-scale directional, downwind flight of whole
swarms is initiated (Culmsee 2002).

This movement can bring the swarms to other
desert arid or hyperarid areas, or to non-desert
areas (Figure 3.7) - the "outbreak areas" (as
distinct from the areas they inhabited prior to the
massive movement - the "recession areas",
Pedgley 1981). Carried by the wind which takes
them to where it takes the rain too (Waloff 1960),
swarms that may contain 50 000 million individuals,
migrate from deserts into non-desert areas, where
they can spread over more than 20 per cent of the land surface of up to 65 countries including
in the Sahara and the Arabian Desert. They
can reach southern Spain, Turkey, West Africa,
India, Bangladesh, Tanzania and the Democratic
Republic of Congo, consuming 100 000 tons of
vegetation a day. Swarms originating in deserts can
even cross oceans; during the 1986-89 plague,
swarms escaped extermination in western Africa
and crossed 5 000 km over the Atlantic Ocean,
reaching the West Indies and the eastern coast of
Venezuela. Swarms migrating to outbreak areas
rarely return to the original recession areas (Ibrahim
and others 2000): they die out, or are destroyed by
cold weather or pest control measures. Since it is
the desert edges that are often the source of locust
plagues, it is there, in countries such as Algeria,
Somalia, Sudan and Iran, where the battle against
locusts can be most-effectively fought, in order
to save crops in other areas, in countries such as
Mali, Niger, Chad and Yemen (Showler 2002).